The present invention relates to a new and improved method of, and apparatus for, deriving a first scanning sequence of first scanned values or signals which are required at a first scanning frequency, from a second scanning sequence of second scanned values or signals which arrive at a second scanning frequency.
The term "approximately synchronous audio systems" is used in the art with respect to two or more digitally operating audio systems employing scanning frequencies which coincide or approximately coincide over an extended time span. Conventionally such audio systems are controlled by means of clock signals. Such clock signals appear at frequencies constituting a multiple of the aforementioned scanning frequencies. Since the clock signals for two or more of such audio systems do not always originate from the same clock signal generator, the clock signals do not always appear at precisely the same moment of time in the two audio systems. As a consequence also the individual scanning sequences at the different scanning frequencies deviate from each other when considered during different time intervals and the individual scanned values or signals do not appear exactly at the same moments of time.
On the receiving side the data cannot be accepted at random moments of time within each scanning interval. Even more so, it must be avoided that data are read-in at moments of time at which they are also required on the receiving side. Therefore, there are frequently used systems which contain staggered buffer stores or which operate with multiple read-in cycles.
When the transmitters and receivers are not exactly synchronized, the slowly changing relative positions of the respective clock signal generators may have the consequence that, for example, a data item or word is lost or repeated in one audio system. For example, if two scanning frequencies having a rated value of 50 kHz deviate from each other by one millionth, one data item or word is lost or repeated every twenty seconds. Such errors are audible in digitally operating audio systems and, therefore, must be avoided. Clearly, such errors do not occur in cases in which all concerned audio systems are supplied or controlled by the same clock signal generator. In practice, however, such solution of the problem frequently is impracticable or too expensive.
The operation of combining approximately synchronous audio systems without an audible error is called "scanning frequency synchronization" in the art. Hitherto, substantially two methods for synchronizing scanning frequencies have become known.
A first solution of the problem was suggested under the title "Sampling Rate Synchronization by Variable Delay" in the BBC Research Report, RD 1979/17, September 1979. This solution consists of supplying one of, for example, two scanning sequences to storage means. During normal operation and corresponding to the so-called FIFO-principle (first in, first out), the first read-in scanned values or signals are also read-out again first. When, during a certain time interval, more scanned values or signals are read-in than read-out, this may result in a storage overflow. In order to prevent an overflow of the storage means, a portion of the storage content is not read-out or is passed-over. When, during a certain time interval, less scanned values or signals are read-in than read-out, predetermined standard scanned values or signals are added during the read-out operation. Both of the aforementioned operations are performed only at the occurrence of low-volume passages in the audio signal. In this manner this operation is not always audible.
A second solution to the problem is known, for example, from European Patent No. 0,052,847 granted July 7, 1985, and Eupean Patent Publication No. 0,137,323, published Apr. 17, 1985, and their cognate commonly assigned and co-pending U.S. patent application Ser. Nos. 06/674,633 and 06/678,776. According to this solution a scanning rate converter is used. By means of such scanning rate converters there can be transformed into each other and over a wide range, scanning sequences of different scanning frequencies.
It is a disadvantage of the first solution that the signal is delayed in the storage means. When a number of such devices are series-connected, this delay may become noticeable in an interfering manner. Furthermore, the appearance of such low-volume signals must be watched for in order to carry out the delay compensation between the scanning sequences. When the storage means is dimensioned too small, the appearance of the low-volume signals cannot be waited for and the correction becomes audible. When the storage means is dimensioned sufficiently large, the average signal delay is correspondingly greater. It must be added that, by using this apparatus, the regular sequence of the individual scanned values or signal is disturbed. This renders difficult the use of a time code or other accompanying data or information. It is, however, an advantage that the arriving signals are not subjected to quantization.
It is also the disadvantage of the second solution that the signal is delayed. In a series-connection of a number of scanning rate converters the noise which is caused by the quantization can appear in an interfering manner. Furthermore, the apparatus expense depending upon the construction, can be significant when such scanning rate converters are used.